Image forming apparatuses typically include a development device to develop latent images formed on a photoreceptor with developer, and two-component developer consisting essentially of toner (toner particles) and magnetic carrier (carrier particles) is widely used in image forming apparatuses. Development devices typically include a development roller serving as a developer bearer and a developer conveyance member to transport the developer inside the development device. Toner in the developer contained in such development devices is consumed in image development, and a toner supply device supplies toner to the development device as required, thereby keeping the concentration of toner in the developer in the development device within a predetermined range.
If the concentration of toner in developer is lower than the predetermined range, image density becomes insufficient, and it is possible that lines or letters in output images become thinner and fade. By contrast, if the concentration of toner in developer is higher, image density becomes excessive, and it can cause bulging of lines and letters or scattering of toner in the backgrounds of output images. Moreover, desired hue cannot be attained if concentrations of respective color toners fluctuate in multicolor image forming apparatuses that use multiple development devices and form multicolor images by superimposing single-color images one on another.
In view of the foregoing, it is necessary to detect the concentration of toner in the developer in the development device and control supply of toner to the development device. For example, concentration of toner in two-component developer can be detected based on magnetic permeability of the developer since magnetic permeability of the developer changes as the concentration of toner therein changes as proposed in JP-2007-034043-A and JP-2010-217328-A.
There are image forming apparatuses that use different processing velocities, that is, the number of copies per minute (CPM). In such image forming apparatuses, the rotational velocity of the development roller of the development device is varied in accordance with the CPM. For example, the developer conveyance member is a conveyance screw including a rotary shaft and spiral-shaped blade winding around the shaft, and transports developer axially by rotation. When the conveyance screw and the development roller are driven by a common drive source, the rotational velocity of the conveyance screw changes as the rotational velocity of the development roller is changed.
In such a configuration, depending on the position where the toner concentration detector detects the toner concentration (i.e., toner concentration detection position), it is possible that changes in the rotational velocity of the conveyance screw affect the magnetic permeability detected. More specifically, the detected magnetic permeability decreases as the rotational velocity of the conveyance screw increases.